1. Field of the Invention
The present invention relates to an exposure apparatus. More particularly, the invention relates to an exposure apparatus having a light source such as an excimer laser, a harmonic laser or a mercury lamp, which emits ultraviolet rays, particularly a spectral beam overlapping with an absorption spectrum of oxygen.
2. Related Background Art
The lithography process for producing semiconductor elements or liquid crystal boards has been employing an exposure apparatus for exposure of an image of pattern on a reticle (e.g., photomask), on a photosensitive substrate through a projection optical system. A recent trend is to develop finer semiconductor integrated circuits. It is thus considered for the lithography process that a method for decreasing the exposure wavelength of the lithography light source is employed as means for enabling formation of finer patterns.
Presently, already developed is an exposure apparatus employing a KrF excimer laser of wavelength 248 nm as a stepper light source. Attention is being given as candidates of shorter wavelength light sources to harmonics of wavelength-variable lasers such as a Ti-sapphire laser, a fourth harmonic of YAG laser of wavelength 266 nm, a fifth harmonic of YAG laser of wavelength 213 nm, a mercury lamp of wavelength near 220 nm, or 184 nm, an ArF excimer laser of wavelength 193 nm, etc.
If an exposure apparatus employs a conventional light source, for example, the g-line, the i-line, the KrF excimer laser, or a mercury lamp emitting light of wavelength near 250 nm, an emission spectrum of such light source never overlaps with an absorption spectral region of oxygen as shown in FIG. 6. Therefore, the oxygen absorption neither lowered the light utilization factor nor caused production of ozone in that case. Accordingly, such exposure apparatus permitted exposure basically in the atmosphere.
A light source such as the ArF excimer laser, however, has an emission spectral band overlapping with the absorption spectral region of oxygen as shown in FIG. 6, so that the oxygen absorption lowers the light utilization factor and causes a problem due to generation of ozone. For example, suppose the transmittance of an ArF excimer laser beam is 100%/m in vacuum or in inert gas such as nitrogen or helium. Then the transmittance in the atmosphere is about 90%/m for free run condition (spontaneous emission state) or for an ArF wide band laser, while about 98%/m even for an ArF narrow band laser with a narrowed spectral band as shown avoiding the absorption lines of oxygen.
It is considered that the decrease in transmittance is caused by absorption of light by oxygen and influence of produced ozone. Not only does the produced ozone negatively affect the transmittance (light utilization factor), but also degrades the performance of apparatus because of the reaction with the surface of optical material or other components, or causes environmental pollution.
It is well known that the entire optical path must be filled with inert gas such as nitrogen to avoid the decrease in transmittance of light or the generation of ozone in the exposure apparatus having a light source such as the ArF excimer laser. An exposure apparatus is generally composed of an illumination optical system for uniformly illuminating a reticle with light from a light source, a projection optical system for focusing a circuit pattern formed on a reticle, on a wafer, and stage means for supporting the wafer and properly moving it for positioning.
In case of the exposure apparatus with the above-described structure, the optical path from the light source to a final optical member (e.g., a lens) in the projection optical system was hermetically concealed in a receptacle filled with inert gas, or each of the illumination optical system, the reticle and the projection optical system was hermetically concealed in a receptacle filled with inert gas.
Since the illumination optical system and the projection optical system each are basically constructed as a stationary unit, the inert gas atmosphere will never be broken in the specific exposure projection process once the inert gas atmosphere is established.
The stage means, however, requires frequent exchange of wafers. Further, the wafer stage must be always two-dimensionally moved to transfer the circuit pattern onto a plurality of exposure areas in a wafer. Thus, if the entire region of from the end portion of projection optical system to the stage means is confined in a receptacle filled with inert gas, the receptacle becomes large in scale and complex. Also, the inert gas atmosphere is broken every exchange of wafers, which requires a considerable time to again establish a desired inert gas atmosphere and which in turn lowers the throughput.
In addition, the inert gas atmosphere is also broken every exchange of reticles in the reticle area, which also needs a considerable time to again establish the desired inert gas atmosphere after evacuation and introduction of inert gas, lowering the throughput. In case a plurality of reticles are used for a single wafer, the frequency of reticle exchange becomes higher so as to further lower the throughput.
Further, there are a lot of members related to alignment of reticle, for example, alignment sensors, around a reticle table on which the reticle is mounted, which could cause a fluctuation of gas flow with re-introduction of inert gas to change an index of refraction of the atmosphere. The index change could induce an alignment error in turn. Therefore, after the re-introduction of inert gas, the alignment of the reticle must wait until the atmosphere is settled, which requires an additional time to re-form the desired inert gas atmosphere.
In case of the conventional exposure apparatus, a material for a reticle case is easily charged and the charge moves to the reticle. This caused such a problem that static electricity appeared on the reticle and the circuit pattern formed on the reticle could be easily broken due to the static electricity.